This invention relates to containers of the so-called "Bliss-type" exemplified by U.S. Pat. No. 1,697,709 to Bliss, dated Jan. 1, 1929, having inside and outside flanges at each corner; the flanges providing stacking strength. Containers of the Bliss-type are formed of three units or blanks of corrugated board or other suitable material; and in the set-up container an inside flange is secured to the inside surfaces of the adjacent front and back walls of the container and an outside flange is secured to the outside surface of the end walls. While the pair of upright flanges at each corner has provided desirable stacking strength, containers of the type illustrated in U.S. Pat. No. 1,697,709 have not been suitable for the storage and transport of granular or powdery substances in bulk since leakage of such substances occurs at the upper box corners.